1. Field of the Invention
The present invention is directed to a power switched amplifier of a type having a switched output stage which generates an output voltage, and a filter stage connected to the switched output stage for smoothing the output voltage. The present invention is further directed to a power switched amplifier of a type suitable for use as a gradient amplifier in a magnetic resonance tomography apparatus.
2. Description of the Prior Art
An amplifier which delivers steep output voltage edges and generally exhibits a low residual ripple, for example, for use as a gradient amplifier of a nuclear magnetic resonance tomography apparatus.
Generally, a nuclear magnetic resonance tomography apparatus has three gradient coils in which exactly regulated periodic currents respectively flow. For example, the current through each gradient coil in a cyclical current curve can reach values up to 300 A that must be adhered to with a precision in the mA range. The current curve can, for example, exhibit a cycle duration of 20 ms, whereby a current rise from0 to 300 A within 1 ms can be required. In order to enable these rapid changes in current, a voltage up to, for example, .+-.300 V having as vertical an edge steepness as possible must be applied to the gradient coil.
Switched gradient amplifiers that, for example, work with a switching clock of 50 kHz are utilized in order to meet these demands, however, an undesired residual ripple (switching ripple) is produced in the output voltage by the switching. The residual ripple may cause resonance in sub-regions of the gradient coil, which is a complex structure with a plurality of local capacitances and inductivities. The high voltages thereby arising can produce local discharges that upset the patient and lead to image disturbances.
German OS 40 07 566 discloses a switched power amplifier of the type initially described. This amplifier has a filter arrangement that contains four assemblies, each with a series inductance and a smoothing capacitor.
In such a filter arrangement, however, the capacitance serving to smooth the output voltage cannot be very large because the maximum edge steepness of the output voltage would otherwise be too greatly diminished. Only a moderate filter effect therefore can be achieved.
The smoothing effect is also dependent on the switching clock frequency of the amplifier. A fast switching clock that exhibits a greater frequency separation from the useful signal is suppressed better by a low-pass filter. Given the high powers to be switched in the circumstances described above, however, an increase in the switching clock is only possible within certain limits and is also very complicated within these limits. A high switching clock causes high losses (that must be eliminated by cooling) and requires the use of more expensive components.